In the provision of so-called soundproof (or sound insulating) structures, it has been the general practice to use one type of construction for fixed walls and another type of construction for doors. However, difficulties have resulted from the doors being far less effective in sound insulation than the fixed walls so that regardless of the sound insulation properties of the wall, much of the efficiency is destroyed by sound transmission through the door.
Another difficulty encountered in connection with sound-insulating door construction results from the fact that some structures, while relatively efficient at preventing the transmission of sound at some frequencies have a poor efficiency at certain particular frequencies.
For many years, the general assumption has been that the control of sound through a barrier, such as a door, is accomplished by absorption. However, doors configured only to absorb sound waves have been found to still allow a significant amount of sound to pass therethrough. More specifically, because sound tends to travel linearly, the sound waves slow down, but still pass through the absorption material and escape through the door.